FIG. 10 is a block diagram showing a configuration of a signal processing unit of a conventional single-CCD digital camera.
A signal from a CCD image sensing element 501 is adjusted in white gain by a white balance (WB) circuit 502, and sent to a luminance notch circuit 510. The luminance notch circuit 510 reduces the gain of color signal components near the Nyquist frequency in the vertical direction by using a vertical low-pass filter (VLPF), and also reduces the gain in the horizontal direction by using a horizontal low-pass filter (HLPF). These filters are called luminance notch filters. Then, a horizontal bandpass filter (HBPF) circuit 511 and vertical bandpass filter (VBPF) circuit 514 shift up frequency components slightly lower than the Nyquist frequency decreased by the notch filters.
Aperture peak (PP) gain circuits 512 and 515 adjust the amplitude in both the horizontal and vertical directions, and base clip (BC) circuits 513 and 516 clip small-amplitude components and remove noise. An adder 517 adds the horizontal and vertical components, an aperture control (APC) main gain circuit 518 applies a main gain to the resultant signal, and an adder 519 adds a baseband signal to the signal. A γ conversion circuit 520 γ-converts the signal, and a luminance correction (YCOMP) circuit 521 corrects the luminance signal level by color.
Further, a color interpolation circuit 503 interpolates a chroma signal so as to give color pixel values to all pixels. A color conversion matrix (MTX) circuit 504 converts the complementary color signals to luminance (Y) signals and color difference (Cr, Cb) signals. Thereafter, a chroma suppress (CSUP) circuit 505 suppresses the color-difference gain in low and high luminance regions, and a chroma low-pass filter (CLPF) circuit 506 limits the band of the chroma signals. A γ conversion circuit 507 converts the band-limited chroma signal into R, G, and B signals and at the same time γ-converts the R, G, and B signals. The γ-converted R, G, and B signals are converted into Y, Cr, and Cb signals again. A chroma gain knee circuit 508 adjusts the chroma gain of the luminance and color difference signals, and a linear clip matrix (LCMTX) circuit 509 finely adjusts the hue as well as corrects shift in hue due to variation in quality of image sensing elements.
Assume that an output from an image sensing element having filters of a checkered Bayer layout as shown in FIG. 4 is processed by the luminance notch circuit 510. In particular, primary color filters achieve good color separation. Therefore, as shown in FIG. 12A, the conventional notch filter method, which is a filtering method using signals from all pixels, cannot absorb gain differences between different color filters by using LPFs at the edge of an image having opposite hues, e.g., red and blue in left and right halves. The edge staircases or becomes jagged, which degrades the image quality of a playback image. This will be explained with reference to FIG. 12B.
FIG. 12B is a view for explaining an output level from each pixel of the image sensing element. In FIG. 12B, a pixel outputting a relatively large value is blank, and a pixel having an output of almost 0 is hatched for descriptive convenience. Signal level differences between different color filters are large at an edge between colors of opposite hues (referred to as “opposite hue edge”, hereinafter), and appear jagged. Further, the jaggedness is enhanced by edge enhancement which is performed to increase the resolution (MTF: Modulation Transfer Function) which has been decreased by LPFs.